Double-focussing mass spectrometer

ABSTRACT

The invention relates to a variable dispersion double-focusing mass spectrometer comprising at least a magnetic sector analyzer (4) preceding an electrostatic analyzer (6), which analyzers cooperate to form a direction and velocity focused image on a multichannel detector (34) locatable in a focal plane of the electrostatic analyzer (6). The geometrical parameters of the electrostatic analyzer are selected so that the magnification of the electrostatic analyzer is substantially zero, which makes it possible to use a variable radius electrostatic analyzer to vary the extent of the mass spectrum imaged on the detector (34) while still maintaining double focusing. A variable radius electrostatic analyzer suitable for use in the invention is also described.

This invention relates to a double focusing mass spectrometerparticularly, though not exclusively; with variable dispersion, and isparticularly useful in connection with a multi-channel detector.

Most conventional high resolution mass spectrometers are of theNier-Johnson or Hintenberger-Konig geometry in which a double (i.e. bothdirection and velocity) focused image is formed at a narrow collectorslit on the ion-optical axis of the last analyzer. In such an instrumenta spectrum is obtained by scanning both the electrostatic and magneticanalyzers to successively focus ions of different mass-to-charge ratioson the collector slit. An ion detector, typically an electronmultiplier, is disposed beyond the collector slit to receive the ionspassing through the slit and produce an electrical signal therefrom.

Although such spectrometers are highly developed and often have highsensitivity and resolution, they are inefficient in so far as only asmall proportion of the ions emitted from a sample are detected at anyone instant during a scan. The efficiency may be improved by the use ofa multichannel detector which is capable of recording a significant partof the spectrum simultaneously. Such detectors typically comprise one ormore microchannel plate electron multipliers followed by a phosphorscreen and either a photodiode array or vidicon television camera fordetecting the position of the electron impacts on the screen. Usually afibre optic coupling is provided between the phosphor screen and thearray or camera.

Multichannel detectors have been fitted to several different kinds ofspectrometers. Dukhanvov, Zelenkov et. al (Instrum. and Expt.Techniques, 1980 vol 23(3) pp 726-9), and Tuithof, Boerboom andMeuzelaar (Int. J. Mass Spectrom. and Ion Phys, 1975, vol 17, pp299-307), describe single focusing magnetic sector instruments fittedwith microchannel plate detectors, and Tuithof, Boerboom, Kistemaker andMeuzelaar (Adv. in Mass Spectrom, 1978, vol 7, pp 838-845) describe amore advanced single focusing spectrometer with a channelplate detectorand variable mass dispersion. Hu, Chen, Boerboom and Matsuda (Int. J.Mass Spectrom and Ion Proc, 1986, vol. 71 pp 29-36) describe a singlefocusing spectrometer with an auxiliary magnet for improvingperformance, and several workers have described Mattauch-Herzogdouble-focusing instruments with such detectors (e.g., Murphy,Mauersberger Rev. Sci. Instrum., 1985 vol. 56 (2) pp. 220-226; andBoettger, Giffin and Norris, A.C.S. symp. ser. No. 102, 1979, pp291-318) Ouwerkerk, Boerboom, Matsuo and Sakurai (Int. J. Mass Spectromand Ion Proc, 1986, vol 70, pp 79-96) and Cottrell and Evans (Anal.Chem. 1987, vol 59(15) pp 1990-1995), report the fitting of multichanneldetectors to double focusing mass spectrometers with Nier-Johnsongeometry.

Serious limitations on performance arise when multichannel detectors arefitted to spectrometers with geometries designed for scanning. Ingeneral, the extent of the spectrum which can be imaged is limited, andresolution is often degraded because of the finite spacing of theindividual channels in the channel electron multiplier or the resolutionof the photodiode array or television camera. The limitations arise atleast in part because the magnification and dispersion of thespectrometer have been selected without regard to the requirements ofthe multichannel detector. Even on the single-focusing instrumentsdescribed by Boerboom and coworkers, the need to provide variabledispersion and magnification to gain full benefit from the multichanneldetector was recognized in 1978, and the limited performance improvementreported for double focusing instruments which have fixed magnificationand dispersion is apparent from the references cited.

In the case of double focusing instruments it is necessary that thevelocity focal plane and the direction focal plane (also known as theenergy and angular focal planes, respectively) are both coincident andsubstantially flat over the extent of the detector. These conditions arenot necessary for a scanning instrument, where the collector slit isvery narrow. They are however characteristic features of Mattauch-Herzogdouble-focusing spectrometers, but unfortunately most such instrumentsare designed for photographic plate detection and the focal plane isboth very extensive and very close to the poles of the magnet. It is notcost effective to build a multichannel detector which extends over theentire focal plane of such an instrument, and consequently the massrange which can be detected is very limited. The performance of thedetector under these conditions is also degraded by the presence of thestray magnetic field. A further disadvantage of the Mattauch-Herzoggeometry is that the spacing of the masses along the focal plane isnon-linear. In the case of Nier-Johnson geometries, the usable extent ofthe focal plane is inherently limited by the physical size of theanalyzers, and may be further reduced because of curvature. Resolutionmay also be limited because the dispersion of the spectrometer is notgreat enough in relation to the channel spacing of the detector.Obviously it is possible to design spectrometers which have adequatedispersion or adequate mass range, but it is impossible to provide bothat once on currently available detectors. In practice, there must alwaysbe a trade off between resolution and mass range, so that for example, asmall part of the spectrum can be simultaneously recorded at highresolution or a much larger part at low resolution, and up to now thatchoice has to be made when the instrument is designed. In order tomaximize the benefit obtainable from a multichannel detector, therefore,it is desirable to have a spectrometer with variable dispersion, similarto that described by Boerboom for a single focusing instrument. However,no such instrument has been described. This is mainly because in allknown double focusing geometries, independent variation of thedispersion is impossible without loss of the double focusing conditions,without which resolution cannot be obtained.

It is an object of the present invention to provide a double focusingmass spectrometer having a multichannel detector in which the dispersioncan be varied without loss of double focusing and by varying only theparameters associated with one of the analyzer sectors. It is a furtherobject of the invention to provide a double focusing mass spectrometerwhich has either selectable or continuously variable mass dispersion. Itis a yet further object to provide such a spectrometer fitted with amultichannel detector which is capable of recording either a substantialpart of the mass range of the spectrometer at low mass resolution or alimited part of the mass range at high resolution while double focusingconditions are maintained.

Viewed from one aspect, the invention provides a mass spectrometercomprising at least a magnetic analyzer and an electrostatic analyzer,through which ions pass in that order, which analyzers cooperate to forma direction and velocity focused image, characterised in that thegeometrical parameters of said mass spectrometer are further selectedsuch that the magnification of said electrostatic analyzer issubstantially zero.

Viewed from another aspect the invention provides a mass spectrometercomprising at least a magnetic analyzer adapted to receive ions formedfrom a sample, and an electrostatic analyzer adapted to receive at leastsome of said ions after they have passed through said magnetic analyzerand to form in cooperation with said magnetic analyzer a direction andvelocity focused image therefrom, characterised in that said magneticanalyzer produces a mass dispersed and direction focused ionic imagelocated substantially at infinity.

Viewed from yet another aspect, the invention provides a massspectrometer comprising at least a magnetic analyzer and anelectrostatic analyzer, through which ions pass in that order, whichanalyzers cooperate to form a direction and velocity focused image,characterised in that the trajectories of ions travelling between saidanalyzers are substantially parallel.

In any such mass spectrometer it can be shown that the conditions fordouble focusing (i.e., the production of a direction-focused andvelocity-focused image) is independent of factors such as the overallmagnification and the distance between the magnetic and electrostaticsectors, in contrast to a conventional double focusing spectrometer withNier-Johnson or Hintenberger-Konig geometry. Consequently, the overallmagnification (and therefore dispersion at the detector focal plane) ofa spectrometer according to the invention can be varied by changing thefocal length and image or object distance of one of the analyzerswithout having to make compensatory adjustments to the dimensions of theother to maintain double focusing.

As explained, a double-focusing spectrometer having easily variable massdispersion is especially valuable when fitted with a multichanneldetector. The invention therefore further provides a mass spectrometersubstantially as described, said electrostatic analyzer beingconstructed to allow its effective radius to be varied, and saidspectrometer further comprising at least one multichannel detectorlocatable in the mass-dispersed image plane of the electrostaticanalyzer at whatever value of the effective radius that is selected,whereby portions of the mass spectrum of the ions entering saidelectrostatic analyzer may be imaged on said detector at differentdispersions according to the selected value of said effective radius.

The term "effective radius" is taken to mean the radius of a circulararc which is tangential to the central trajectory of the ions at thepoints where they enter and leave the electrostatic field, irrespectiveof the actual shape of the trajectory through the analyzer. It will beappreciated that the position of the multichannel detector will varywith the selected radius of the electrostatic analyzer, and thereforetwo or more detectors may be provided, one at each of the image planesassociated with a particular effective radius. The first of thesedetectors must obviously be retractable to allow ions to pass to thesecond detector when required. Alternatively, one detector, translatablebetween two or more positions, may be provided.

In an alternative embodiment, an electrostatic lens means of variablefocal length may be provided between the exit of the electrostaticanalyzer and a single multichannel detector. The lens means is adaptedto form a focused image on the detector from the intermediate image ofthe mass spectrum produced at a particular distance from the analyzer bythe analyzer at the selected value of the effective radius. By suitableadjustment of the focal length of the lens means, a focused image may beprojected onto the detector, irrespective of the position of the imageproduced by the electrostatic analyzer. In the following, references tolocating the detector in the focal plane of the analyzer are meant toinclude the use of a lens in this way.

In order to obtain full benefit from the invention, an electrostaticanalyzer with variable or selectable effective radius is required. Suchan analyzer can be constructed in several ways. In one embodiment, theelectrostatic analyzer may comprise a plurality of individual analyzersegments, each of different effective radii, in which the effectiveradius is selected by applying appropriate potentials to any selectedone of the segments. Obviously the segments must be such that the ionbeam can pass undeflected through them when they are not energized,e.g., by ensuring that the gap between the electrodes of each segment issufficiently large in relation to their curvature to permit this. Itwill be appreciated that this arrangement may be double focusingwhichever of the individual segments is operational, because in aspectrometer according to the invention the conditions for doublefocusing are independent of the distance between the magnetic andelectrostatic analyzers.

In a more preferred embodiment the electrostatic analyzer may comprise acentral segment and one or more pairs of outer segments respectivelydisposed one on each side of the central segment. The central segmentmay comprise an analyzer of a first effective radius and each pair ofouter segments are arranged in conjunction with the central segment, andany others of the outer segments between its segments and the centralsegment, to comprise an analyzer with a second effective radius havingsubstantially the same sector angle as the first analyzer. Preferablythe outer segments are symmetrically disposed about the central segment.

Each analyzer segment may comprise pairs of cylindrical sector, toroidalsector or straight plate electrodes disposed one on each side of the ionbeam in the manner of conventional single-segment analyzers. Mostconveniently, each segment may comprise a pair of substantially parallelstraight electrodes. All the electrodes on the same side of the ion beamare preferably disposed in the same plane, so that the complete analyzercomprises two parallel straight electrodes disposed one on each side ofthe ion team with each electrode divided into segments. Typically theelectrodes of the central segment will be different lengths, therebydefining a sector angle, and the electrodes comprising an outer segmentwill be of equal lengths so that an analyzer comprising the centralsegment and two symmetrically disposed outer segments will have the samesector angle as that comprising only the central segment but a largereffective radius.

Alternatively, especially if a large deflection angle is required, theelectrodes of the outer segments may be inclined with respect to theelectrodes of the central segment so that the physical disposition ofthe electrodes resembles a cylindrical sector analyzer in which theelectrodes each comprise several straight electrodes of relatively shortlength.

In further preferred embodiments, at least one of the segments of theanalyzer may comprise a pair of main electrodes disposed one on eachside of said ion beam and two groups of auxiliary electrodesrespectively disposed above and below the beam and spaced apart betweenthe main electrodes. Typically the auxiliary electrodes are the sameshape as the main electrodes (e.g. straight plates in the case of a"parallel-plate" segment or circular arcs in the case of a cylindricalsector), and are equally spaced between the main electrodes. The upperand lower groups of electrodes may be substantially identical,comprising the same number, type and spacing of electrodes.Corresponding electrodes in each group may then be electricallyconnected so that there is no electrical field along an axisperpendicular to the planes of the auxiliary electrodes (i.e., the "z"axis of the analyzer), as in a conventional cylindrical sector analyzer.Each pair of auxiliary electrodes is held at a different potentialthereby defining the electrostatic field in the analyzer segment. In theabsence of the auxiliary electrodes, the potential between the twoparallel straight electrodes would vary linearly with the distancebetween them, and if the auxiliary electrode potentials are selected tocorrespond with this variation, their only effect will be to reduce theeffect of fringing fields due to the analyzer vacuum housing which mightotherwise penetrate between the main electrodes and destroy the fieldhomogeneity. When used in this way the auxiliary electrodes serve auseful purpose in allowing the main electrodes to be separated by agreater distance without trouble from fringing fields, which in turnallows a greater part of the mass spectrum to be simultaneously imagedon a focal plane detector.

Another important use of the auxiliary electrodes is to permit thehomogeneity of the field between the main electrodes to be varied simplyby adjusting the electrode potentials. For example, the potentialbetween the main electrodes may be set to vary according to thepolynomial equation

    V.sub.E =V.sub.M +V.sub.A x.sub.E +V.sub.B x.sub.E.sup.2 +V.sub.C x.sub.E.sup.3 +. . .

wherein V_(E) is the potential at an auxiliary electrode located at adistance x_(E) from the central trajectory of the analyzer, V_(M) is thepotential of the central electrode, and V_(A), V_(B) and V_(C) areconstants selected as required. In this way, second and third ordervariations in homogeneity can be introduced into the field between themain electrodes simply by application of appropriate potentials to theauxiliary electrodes, and these variations can be used to optimizefocusing of a complete analyzer. Most importantly, the constants V_(A),V_(B) and V_(C) can easily be varied for each segment to optimize thefocusing whatever radius is selected. The constants may also be selectedto obtain optimum focusing when the complete analyzer is usedconventionally for ions of constant energy, or for the analysis of ionsof constant velocity, such as fragment ions produced in a collision celllocated between the magnetic analyzer and electrostatic analyzer of acomplete spectrometer.

The auxiliary electrode pairs may conveniently be supplied from apotential divider network comprising suitably selected resistor values,or may be individually supplied from computer controlleddigital-analogue converters if many different sets of coefficients arerequired.

It will be appreciated that if the auxiliary electrode structure extendssufficiently far from the central trajectory of the analyzer it ispossible to omit the main electrodes, and define the electrostatic fieldwithin the analyzer segment solely by means of the potentials applied tothe auxiliary electrodes. Obviously, omission of the main electrodeswill result in severe fringing fields at the ends of the electrodestructure, but providing that a sufficient number of electrodes areprovided, it may still be possible to define the field in the vicinityof the ion beam sufficiently accurately.

It will also be appreciated that the auxiliary electrodes should be asthin as possible to minimize the length of "constant potential" in theanalyzer field in the vicinity of each electrode, and their spacingshould be small enough to ensure that the deviation from the idealpotential gradient between the electrodes is not large enough to causesignificant aberrations.

The most preferred form of analyzer for use in a spectrometer accordingto the invention therefore comprises a multi-segment analyzer havingparallel straight electrodes, and sets of auxiliary electrodes for eachof the segments of the analyzer.

In this way a high resolution double focusing mass spectrometer withmultichannel detection and continuously variable or selectable massdispersion is provided, allowing maximum advantage to be gained from theuse of a multichannel detector.

A preferred embodiment of the invention will now be described byreference to the following figures, in which:

FIG. 1 is a schematic drawing of a spectrometer according to theinvention;

FIG. 2 is a schematic diagram of an electrostatic analyzer suitable foruse in the spectrometer of FIG. 1;

FIG. 3 is a schematic drawing of the spectrometer of FIG. 1 showing theresolution of the ion beam into high and low mass components; and

FIG. 4 is a sectional view of an electrostatic analyzer suitable for usein the spectrometer of FIGS. 1 and 3, along the plane A--A shown in FIG.3.

Referring first to FIG. 1, an ion source shown schematically at 1generates an ion beam 2 which passes through a source slit 3. Beam 2passes through a magnetic sector analyzer 4 comprising a magnet whichdeflects the ion beam according to the mass-to-charge ratios of theconstituent ions. Ions of a selected mass-to-charge ratio leave themagnetic sector analyzer in a substantially parallel beam 5 and enter anelectrostatic analyzer 6 which as well as providing energy filtering,focuses the ions into a beam 7 which forms an image at the collectorslit 8. An ion detector 9 receives the ions after they have passedthrough slit 8. Alternatively, a multichannel detector may be providedin the place of the slit 8.

In the following, the conventional system of co-ordinates is used, i.e.,x is the direction of motion of the ions, y is the dispersion axis ofthe analyzers, (perpendicular to x), and z is the axis perpendicular toboth x and y.

Defining

y₀ =positional displacement of an ion leaving source slit 3,

y₀ '=angular displacement of an ion leaving source slit 3,

y₁ =positional displacement of an ion entering the first analyzing field(i.e., that due to magnet 4, and

y₁ '=angular displacement of an ion entering the first analyzing field,

    then

    y.sub.1 =y.sub.0 +1'y.sub.0 '                              (1)

    and

    y.sub.1 '=y.sub.0 '                                        (2)

where 1' is the distance between the slit 3 and the start of the firstanalyzing field.

Following conventional procedures, the first order transfer matrix whichpredicts the position and angular displacements of the ion as it leavesthe first analyzing field (y₂ and y₂ ' respectively) is ##EQU1## inwhich β represents the fractional velocity displacement (i.e., of theion) and A₁₁ -A₂₃ are matrix elements determined by the geometry of themagnetic field (see below). Consequently, ##EQU2## At the point whereions enter the second analyzing field, typically the electrostaticanalyzer 6, the positional and angular displacements (y₃ and y₃ 'respectively) are given by

    y.sub.3 =y.sub.2 +dy.sub.2 '                               (5)

    and

    y.sub.3 '=y.sub.2 '(6),

where d is the distance between the first and second analyzing fields(see FIG. 1).

At the end of the second analyzing field the positional and angulardisplacements (y₄, Y₄ ') are given by equations (7) and (8), which arederived from a matrix similar to that for the first analyzing field butincorporating the elements B₁₁ -B₂₃ in place of A₁₁ -A₂₃. Elements B₁₁-B₂₃ are related to the geometry of the second analyzing field (seebelow).

    y.sub.4 =B.sub.11 y.sub.3 +B.sub.12 y.sub.3 '+B.sub.13 β(7)

    and

    y.sub.4 '=B.sub.21 y.sub.3 +B.sub.22 y.sub.3 '+B.sub.23 β(8)

Finally, at the collector slit 8, the positional and angulardisplacements are given by

    y.sub.5 =y.sub.4 +1"y.sub.4 '                              (9)

    and

    y.sub.5 '=y.sub.4 '                                        (10)

where 1" is the distance between the end of the second analyzing fieldand collector slit 8 (see FIG. 1).

In general terms, the condition for single-focusing is that y₅ =0 whenβ=0 and y₀ '≠0, and the condition for double focusing is that y₅ =0 whenβ≠0 and y₀ '≠0.

According to a preferred embodiment of the invention, the source slit 3is positioned such that the trajectories of the ions comprising beam 5are substantially parallel, so that the image produced by the firstanalyzing field is located substantially at infinity. In this situation,y₂ ' must be independent of y₀ ' when β is zero, so that from equations(1) and (4),

    A.sub.21 1'+A.sub.22 =0                                    (11)

    and

    y.sub.2 '=A.sub.23 β                                  (12).

Equation (11) defines a general relationship between the object distance1' and the geometrical parameters of the first analyzing field whichshould be satisfied for a spectrometer according to the invention togive a first order focus.

Considering next the second analyzing field, preferred embodiments ofthe invention are such that the second analyzing field receives theparallel beam 5 (i.e., its object is located substantially at infinity)and forms an image at the collector slit 8. At slit 8, y₅ =0, so thatfrom equation (9),

    y.sub.4 +1"y.sub.4 '=0                                     (13)

From equations (6) and (7), and because when β=0,

    y.sub.3 '=y.sub.2 '=0,

    y.sub.4 =B.sub.11 y.sub.3 +B.sub.12 y.sub.3 '=B.sub.11 y.sub.3(14)

and from equation (8),

    y.sub.4 '=B.sub.21 y.sub.3 +B.sub.22 y.sub.3 '=B.sub.21 y.sub.3(15)

substituting in equation (13)

    B.sub.11 y.sub.3 +1"B.sub.21 y.sub.3 =0

    so that

    B.sub.11 +1"B.sub.21 =0                                    (16)

Equation (16) defines the general relationship between the imagedistance 1" and the geometrical parameters of the second analyzing fieldwhich should be satisfied for a spectrometer according to the inventionto give a first order focus.

Further preferred embodiments of the invention provide spectrometerswith double focusing in addition to the single focusing conditionsdescribed above. For a spectrometer according to the invention to bedouble focusing the condition y₅ =0 when β≠0 must also be satisfied.

Assuming y₁ =0 and y₁ '=0, then from the previous equations, ##EQU3##From (7),

    y.sub.4 =(B.sub.11 (A.sub.13 +dA.sub.23)+B.sub.12 A.sub.23 +B.sub.13)β

and from (8),

    y.sub.4 '=(B.sub.21 (A.sub.13 +dA.sub.23)+B.sub.22 A.sub.23 +B.sub.23)β.

The condition for first order focusing is

    y.sub.5 =y.sub.4 +1"y.sub.4 '=0, so that ##EQU4## Equation (16) must also be satisfied for first order focusing, i.e.,

    (B.sub.11 +1"B.sub.21)=0                                   (16)

so that ##EQU5##

Equation (17) defines the relationship between the geometricalparameters of the analyzing fields which should be satisfied for aspectrometer according to the invention to be double focusing. It willbe seen that the condition is independent of d so that both the singlefocusing and double focusing conditions are independent of the distancebetween the analyzing fields.

Following conventional practice, the coefficients A₁₁ -A₂₃ and B₁₁ -B₂₃can be written as a set of dimensionless coefficients a₁₁ -a₂₃ and b₁₁-b₂₃, with, for example, the factor (where present) which scales thecoefficient relative to the size of the analyzing field separated out.For example, if the trajectories of the ions through both fields iscircular, the coefficients may be written as below: ##EQU6## where r_(a)and r_(b) are the effective radii of the trajectories through the firstand second analyzing fields, respectively. (The validity of thisrepresentation will be made clearer by consideration of the equationsfor the various coefficients given below for specific analyzers). If thesecond analyzing field is a parallel plate electrostatic analyzer, inwhich the ion trajectory is parabolic rather than circular, then r_(b)is simply replaced by 1_(b) (the length of the analyzer plates).

Substituting in equation (17) ##EQU7## Therefore, providing thecondition ##EQU8## is met, double focusing occurs irrespective of thevalues of r_(a), r_(b) and d.

Considering next the mass dispersion of a spectrometer according to theinvention, the transfer matrix for the first analyzer A can be written##EQU9## in which γ is Δm/m, so that at the exit of the first analyzer,

    y.sub.2 =A.sub.11 y.sub.1 +A.sub.12 y.sub.1 '+A.sub.13 β+A.sub.14 γ

    and

    y.sub.2 '=A.sub.21 y.sub.1 +A.sub.22 y.sub.1 '+A.sub.23 β+A.sub.24 γ

Assuming as in the previous discussion that

    y.sub.0 =y.sub.0 '=β=y.sub.1 =y.sub.1 '=0,

    then

    y.sub.2 =A.sub.14 γ and y.sub.2 '=A.sub.24 γ.

At the entrance of the second analyzer

    y.sub.3 =y.sub.2 +dy.sub.2 '=(A.sub.14 +dA.sub.24)γ

    and

    y.sub.3 '=y.sub.2 '=A.sub.24 γ

The transfer matrix for the second analyzer B can be written ##EQU10##

    so that

    y.sub.4 =B.sub.11 (A.sub.14 +dA.sub.24)γ+B.sub.12 A.sub.24 γ+B.sub.14 γ

    and

    y.sub.4 '=B.sub.21 (A.sub.14 +dA.sub.24)γ+B.sub.22 A.sub.24 γ+B.sub.24 γ

    At the collector slit 8

    y.sub.5 =y.sub.4 +1"y.sub.4 ',

    so that ##EQU11##

Substituting equation (16) in equation (18),

    y.sub.5 =[(B.sub.12 -B.sub.11 /B.sub.22 B.sub.21)A.sub.24 +(B.sub.14 -B.sub.11 B.sub.24 /B.sub.21)]γ                     (19)

Replacing the coefficients B₁₁ -B₂₃ in (19) with the dimensionlesscoefficients listed previously and noting that a₂₄ =A₂₄ and b₂₄ =B₂₄,

    y.sub.5 =[(b.sub.12 -b.sub.11 b.sub.22 /b.sub.21)a.sub.24 +(b.sub.14 -b.sub.11 b.sub.24 /b.sub.21)]r.sub.b γ             (20)

From equation (20) it can be seen that in the general case, y₅(effectively the mass dispersion) is related to the radius of the secondanalyzer r_(b) and is independent of r_(a) and d. When the firstanalyzer is a magnet and the second analyzer is an electrostaticanalyzer, then the coefficients B₁₄ =b₁₄ r_(b) =0 and B₂₄ =b₂₄ =0, sothat equation (20) simplifies to

    y.sub.5 =(b.sub.12 -b.sub.11 b.sub.22 /b.sub.21)a.sub.24 r.sub.b γ

It is this property of a spectrometer according to the invention thatallows a variable dispersion double focusing instrument to beconstructed in practice simply by varying the effective radius of thesecond analyzing field (i.e., the electrostatic analyzer), and adjustingthe position of the detector accordingly (equation 16). Clearly, at aparticular value of the effective radius of the electrostatic analyzer aparticular portion of the mass spectrum is simultaneously imaged on thedetector with a particular dispersion and therefore resolution. Bychanging the effective radius and with it the dispersion, either a largeportion of the spectrum can be imaged at low resolution or a smallerportion imaged at high resolution. This maximises the benefit ofmultichannel detection.

For a magnetic sector homogeneous field analyzer, the values of thecoefficients a₁₁ -a₂₄ are related to the geometrical parameters asfollows: ##EQU12## where φ_(m),ε' and ε" are the sector angle of themagnetic analyzer and the inclination of its pole faces (see FIG. 1 forthe precise definition).

For a cylindrical electrostatic analyzer, the coefficients b₁₁ -b₂₄ aregiven by ##EQU13## in which φ_(e) is the sector angle of theelectrostatic analyzer.

For a parallel plate type of electrostatic analyzer, the coefficientsb₁₁ -b₂₄ are given by ##EQU14## Similar expressions for other types ofanalyzers can be obtained from standard texts on their design. Theexpressions for these coefficients clearly show how the values of r_(a)and r_(b) can be extracted from the coefficients A₁₁ -A₂₄ and B₁₁ -B₂₄to yield the dimensionless coefficients a₁₁ -a₂₄ and b₁₁ -b₂₄, whichdepend only on the sector angles φ_(m) and φ_(e) and the pole faceinclinations ε' and ε".

Because the double focusing condition is also independent of thedistance between analyzers, it is possible to construct a variableradius electrostatic analyzer in the following way. Referring to FIG. 2,an electrostatic sector analyzer suitable for use in the inventioncomprises a central segment (electrodes 13 and 18), and two pairs ofouter segments (electrodes 12, 17, 14, 19 and 11, 16, 15, 20respectively). The electrodes are disposed symmetrically about a centreline 31, as shown. Electrodes 11, 15, 16 and 20 are generally groundedand used only as guard electrodes. The lengths of the central segmentelectrodes 13 and 18 are selected to define a parallel plate typeanalyzer of effective radius r_(e), and sector angle φ_(e) with thefield boundaries approximately indicated by lines 21 and 22. When thisradius is selected, electrodes 12, 14, 17, and 19 are also grounded andelectrodes 13 and 18 are energized with appropriate voltages. Ionstravelling along the central trajectory 23 of ion bean 5 thereforecontinue along a straight trajectory 24 until they enter theelectrostatic field at line 21 and then continue along a curvedtrajectory 25 of effective radius r.sub. e1. They leave the field atline 22 to continue along the straight trajectory 26 and the centraltrajectory 27 of ion beam 7.

When radius r_(e2) is selected, the outer segment comprises electrodes12, 14, 17 and 19. Electrodes 12, 13 and 14 are maintained at a firstpotential and electrodes 17, 18 and 19 at a second potential, in orderto define an electrostatic field bounded approximately by lines 28 and29 and having a sector angle φ_(e) and radius r_(e2). Ions enter alongtrajectory 23 until line 28 is reached, then continue along curvedtrajectory 30 (of effective radius r_(e2)) until they reach line 29,leaving along trajectory 27 as before. Lines 21 and 28, and 22 and 29,are parallel so that the sector angle φ_(e) is the same whichever valueof r_(e) is selected, (this is necessary because the coefficients b₁₁-b₁₃ are all dependent on φ_(e)). It will be noted that lines 21 and 28(which define the start of the field) are spaced apart but this does notmatter because the focusing conditions are independent of d, unlike aconventional double focusing spectrometer. Similarly, the electrostaticfield terminates in a different place, depending on which radius isselected, but this is easily allowed for in calculating 1".

It will be appreciated that values of r_(e) intermediate between r_(e1)and r_(e2) can be obtained using an electrostatic analyzer according toFIG. 2 simply by maintaining the outer segment electrodes 12, 14, 17 and19 at suitable potentials intermediate between ground and those requiredfor operation at radius r_(e2). This situation arises because neitherthe position of the field boundaries (indicated by lines 28, 21, 22 and29), nor the actual shape of the trajectory, have any effect on thedouble focusing properties of a spectrometer according to the invention.Consequently, electrostatic analyzers having more electrodes than shownin FIG. 2 can be constructed, and the easy adjustment of the value ofr_(e1) simply by changing electrical potentials can be used to "focus"the mass spectrum exactly at a particular position of the detector.

The practical construction of an electrostatic analyzer according toFIG. 2 presents no difficulty and is in fact less critical in alignmentthan the conventional electrostatic analyzers currently used in doublefocusing mass spectrometers.

In a further variation, the electrodes 11-15 and 16-20 may be disposedtangentially around two circular arcs centered on the point from whichthe effective radii r_(e1) and r_(e2) are measured, thus forming ananalyzer which is a hybrid between a cylindrical analyzer and a parallelplate analyzer. Although the exact values of the coefficients b₁₁ -b₂₃will not be known for such an analyzer, this is of no significancebecause the value of the radius can easily be changed electrically.

Referring next to FIG. 4, an electrostatic analyzer suitable for use inthe invention is enclosed in a vacuum housing 35 closed by a lid 36sealed with an `O` ring 37 and secured by bolts 38. A port 39, closed byan `O`-ring sealed flange 40 which carries a number of electricalfeedthroughs 41, is provided to allow electrical connection to be madeto the electrodes comprising the analyzer (e.g., lead 42).

The section shown in FIG. 4 is taken through the central segment of theanalyzer (i.e., plane A--A in FIG. 3), but the other segments of theanalyzer are of substantially identical construction.

The main electrodes 13 and 18 of the central segment comprise straightplates of length selected to define the required sector angle φ_(e) aspreviously explained.

They are supported on four insulated mountings 43 (two for eachelectrode) from brackets 44 which are secured to the floor of the vacuumhousing 35 with screws 45. Each electrode (13 or 18) is spaced apartfrom brackets 44 by a ceramic tube 46 and is secured by a screw 47fitted with a ceramic sleeve 48, and a short ceramic tube 49 is fittedunder the head of screw 47 as shown.

An upper group 50 and a lower group 51 of auxiliary electrodes (e.g.,52) are each supported on two ceramic rods 53 which are located in holesdrilled in the main electrodes 13 and 18. The auxiliary electrodes 52are spaced apart by ceramic bushes 54. Each electrode 52 consists of athin (e.g. 0.5 mm) rectangular metallic plate approximately the samelength as the main electrodes. The height of the electrodes should beseveral, preferably five to ten, times their spacing for the effect offringing fields to be negligible.

In order to minimize the number of electrical connections to theauxiliary electrodes, corresponding electrodes in the upper group 50 andthe lower group 51 are connected together. Similarly, the auxiliaryelectrodes in the outer segments, also disposed in housing 35, aresimilarly connected. To further reduce the number of feedthroughsneeded, all the auxiliary electrodes associated with the segmentcomprising main electrodes 12 and 17 are internally connected to thecorresponding auxiliary electrodes associated with main electrodes 14and 19, so that only 11 feedthroughs are required for the auxiliaryelectrodes of the central segment and a further 11 for all the auxiliaryelectrodes of the surrounding segments. As explained above, all theelectrodes associated with the extreme outer segments (comprising mainelectrodes 11, 15, 16 and 20) are grounded and require no feedthroughsat all. Thus although the complete analyzer comprising 5 segments has110 auxiliary electrodes, only 22 feedthroughs are required in total(plus 4 for the main electrodes).

Each of the two sets of auxiliary electrodes (i.e., the central segmentand the symmetrical outer segments) are fed from potential dividernetworks comprising resistors selected to obtain the desired potentialgradient between the main electrodes. The potential of the centralelectrode is of course ground potential (assuming that the entrance slitof the analyzer is also at ground potential, as is conventional), andthe two main electrodes 18 and 13 are respectively positive and negativewith respect to ground, as they would be in a conventional analyzer.This method of feeding the electrodes is well known. In order to changethe potential gradient, the electrodes are simply switched to adifferent pair of potential dividers.

Several types of multichannel detector are suitable for use in aspectrometer according to the invention, and need not be described indetail. Conveniently, one or more channelplate electron multipliers maybe provided, followed by a phosphor screen. Light emitted by thephosphor is transmitted through a coherent fibre optic bundle to aposition sensitive photodetector such as an array of photodiodes. Suchdetectors are well known. Preferably at least one other detector (notmultichannel) is provided off axis to the main detector. Ions aredeflected into this by means of a deflector electrode, again in aconventional manner.

Referring next to FIG. 3, beam 2 comprises ions of two different m/eratios which are separated by the magnet 4 into two mass resolved beams32 (high mass ions) and 33 (low mass ions) which are focused atdifferent points on a multichannel detector 34. The detector must bealigned with the focal plane of the spectrometer so that both beams 32and 33 are focused simultaneously. In general, the focal plane is not at90° to the axis, in common with conventional spectrometers, but therequired angle can be calculated following conventional procedures fromthe basic focusing equations given earlier. Unfortunately, theinclination of the focal plane varies with different values of r_(e).This can be allowed for by providing a mechanism to adjust the face ofthe detector to the correct angle at each selected image distance 1",but as focal plane tilt is effectively a second order aberration it canbe substantially eliminated by adjustment of the auxiliary electrodepotentials so as to correct the tilt. Similarly, focal plane curvature,a third order aberration, can be corrected by introducing a third ordercomponent into the potential gradient as required. It is difficult todirectly calculate the values of the electrode potentials required, andthe most practical method of selecting them is to use a computer programfor "ray tracing" in ion optical systems. By plotting a group of iontrajectories for a given set of electrode potentials the angle andcurvature of the focal plane can be estimated, and the most suitablevalues of potentials chosen by trial and error. Final adjustment of thepotentials may then be made on a complete spectrometer, for example bytrimming the individual resistor values in the potential divider tomaximize resolution across the entire focal plane. In practice, at anyselected value of r_(e), it may be desirable to provide some focal planerotation by means of the auxiliary electrodes and also to physicallyrotate the detector rather than rotating the focal plane to the desiredposition solely by means of the auxiliary electrodes. This may preventother aberrations becoming too large and reducing the resolution.

Construction of a mechanism for moving a single detector between thevarious positions corresponding to the selected values of r_(e) presentsno special difficulty.

As an alternative to moving a single detector between several positions,two or more detectors may be provided at the desired locations. Meansare also provided for retracting the detectors which are not in use toallow the ion beam to pass to the chosen detector. Retractable detectorsare also known in the art.

I claim:
 1. A mass spectrometer comprising at least a magnetic analyzer, an electrostatic analyzer, through which ions pass in that order, and ion detector means for detecting at least some of said ions, said magnetic and electrostatic analyzers cooperate to form a direction and velocity focused image, said mass spectrometer having geometrical parameters such that the magnification of said electrostatic analyzer is substantially zero.
 2. A mass spectrometer according to claim 1 in which the effective radius of said electrostatic analyzer (6) may be varied, said spectrometer further comprising at least one multichannel detector (34) locatable in the mass dispersed image focal plane of said electrostatic analyzer (6) at whatever value of effective radius that is selected, whereby portions of the mass spectrum of the ions entering said electrostatic analyzer may be imaged on said detector at different dispersions according to the selected value of said effective radius.
 3. A mass spectrometer according to claim 1 in which said electrostatic analyzer (6) comprises a plurality of individual analyzer segments (11-20), each of different effective radii, in which the effective radius of said electrostatic analyzer (6) is varied by applying appropriate potentials to the electrodes comprising any selected one of said analyzer segments.
 4. A mass spectrometer according to claim 3 in which at least one of said segments comprises two groups (50, 51) of spaced-apart electrodes (52) respectively disposed above and below the beam of ions entering said segment, the potentials of the electrodes comprising each said group progressively increasing from one electrode (52) to the next, thereby providing an electrostatic field in a plane between said groups of electrodes which is capable of deflecting said ions along different curved trajectories according to their energy.
 5. A mass spectrometer according to claim 4 in which said groups (50, 51) of electrodes (52) are substantially identical and each electrode in one of said groups is maintained at the same potential as the electrode in a corresponding position in the other of said groups.
 6. A mass spectrometer according to claim 4 in which each of the electrodes (52) comprising each group (50, 51) is maintained at a potential V_(E) given by a polynomial expression of the form

    V.sub.E =V.sub.M +V.sub.A x.sub.E +V.sub.B X.sub.E.sup.2 +V.sub.C X.sub.E.sup.3 +. . .

where V_(E) is the potential applied to a particular electrode, V_(M) is the potential of the central electrode, x_(E) is the distance of that electrode from the central trajectory (positive in one direction, negative in the the other direction), and V_(A), V_(B), and V_(C) are constants.
 7. A mass spectrometer according to claim 6 in which the constants V_(A), V_(B) and V_(C) are chosen to reduce second and third order aberrations in the image formed by said electrostatic analyzer (6).
 8. A mass spectrometer according to claim 6 further comprising a multichannel detector (34) and in which the constants V_(A), V_(B) and V_(C) are selected to result in alignment of the image focal plane of said electrostatic analyzer with the surface of said detector for at least a substantial portion of the length of said detector, at least at one selected value of said effective radius.
 9. A mass spectrometer according to claim 4 in which said electrostatic analyzer (6) can be set to at least two different effective radii, said groups (50, 51) of electrodes (52) are provided for at least three of said segments, and all said electrodes (52) comprised in said groups (50, 51) are maintained at a first set of potentials when one said radius is selected and a second set of potentials when the other said radius is selected, said first and second sets of potentials being respectively selected to optimize the resolution of said spectrometer when said first or said second radius is selected.
 10. A mass spectrometer according to claim 3 in which each said segment comprises two substantially parallel, straight main electrodes (13, 18) intersected by and extending on both sides of a plane in which the ions travel, between which electrodes a potential difference is maintained thereby providing in said plane an electrostatic field capable of deflecting said ions along different curved trajectories according to their energy, and in which all said main electrodes on the same side of the beam of ions in said segment are disposed in a common plane.
 11. A mass spectrometer according to claim 3 in which at least one of said segments comprises a pair of main electrodes intersected by and extending on both sides of a plane in which the ions travel, between which pair of main electrodes a potential difference is maintained, and two groups of auxiliary electrodes respectively disposed above and below said central plane and spaced apart between said main electrodes.
 12. A mass spectrometer according to claim 11 in which said auxiliary electrodes (52) are shaped so that each is spaced a constant distance from said main electrodes.
 13. A mass spectrometer according to claim 1 in which said electrostatic analyzer (6) comprises two or more segments through which ions pass in turn and in which at least one of said segments comprises a first analyzer of a first effective radius and at least one other of said segments in conjunction with the segments comprising said first analyzer comprise a second analyzer of a second effective radius.
 14. A mass spectrometer according to claim 13 in which said electrostatic analyzer (6) comprises a central segment (13, 18) and one or more pairs of outer segments (12, 14, 17, 19) disposed so that ions travel in turn through one segment of each said outer segment pair, the central segment, and the other segment of each said outer segment pair, in which said central segment comprises an analyzer of a first effective radius and each said pair of outer segments (12, 17 or 14, 19) is arranged in conjunction with said central segment, and any others of said outer segments between its segments and said central segment, to comprise an analyzer of a second effective radius and having substantially the same sector angle as the analyzer comprising said central segment alone.
 15. A mass spectrometer comprising at least a magnetic analyzer for receiving ions formed from a sample and to produce therefrom a mass dispersed and direction focused ionic image located substantially at infinity, an electrostatic analyzer for receiving at least some of said ions after they have passed through said magnetic analyzer and to form in cooperation with said magnetic analyzer a direction and velocity focused ionic image therefrom, and ion detector means for detecting at least some of said ions.
 16. A mass spectrometer comprising at least a magnetic analyzer, an electrostatic analyzer, through which ions pass in that order, and ion detector means for detecting at least some of said ions, said magnetic and electrostatic analyzers cooperate to form a direction and velocity focused image, in which said ions travelling between said analyzers have trajectories which are substantially parallel to each other. 